Lead silicate pigments and methods of making same



Patented June 26, 1945 LEAD SILICATE PIGMENTS AND METHODS OF MAKIN GSAME Louis E. Barton, Windsor, Conn.

No Drawing.

Application January 24, 1942, Serial No. 428,035

13 Claims. (Cl. 23-112) In my c'o-pending application Serial No. 359,769, filed October 4, 194,I have disclosed that useful opaque lead silicate pigments can be made by heating a mixture of lead oxide, or-a lead compound which upon heating yields lead oxide. and silica (silicon dioxide, SiOa) under controlled temperatureconditions whereby the reaction to form lead silicate takes place in the solid phase without fusion, thus ensuring a soft fine texture and other desirable physical qualities in the pig- States Patents 2,233,042, February 25, 1941, and

2,236,051, March 25, 1941, I have disclosed methods of stabilizing lead silicate pigments against such discoloration which consist in adding small amounts of alkaline earth halides and alkaline earth oxides, respectively,'to the mixtures of lead and silicon compounds before heating to combine them in the solid phase. In the former patent referred to I have also disclosed the acceleratin eifect upon the rate of reaction when alkaline earth halides are used and the superior quality of the resulting pigment products.

While such stabilizing and reaction-accelerating methods are very efiective the processes have the disadvantage that if more than very small amounts of the stabilizing and accelerating compounds are used it is desirable to thoroughly wash the calcined pigments to remove watersoluble compounds remaining or which may be formed by reaction during calcination. Some of such compounds-for example, alkaline earth oxides-are difiicultly soluble, requiring considerable time and much water to eliminate them from the product. 7 v

I have now discovered a process of accelerating the reaction in the formation of essentially pure lead silicates in the solid phase, whereby lightstable products are formed which are'substantially free from or do not contain water-soluble compounds. My process in brief consists in making a mixture of such lead, silicon and halogen compounds as'will upon heating supply to the product only the oxides of lead and silicon and the halogen, and then heating the mixture until the oxides combine in the solid phase to form an essentially pure lead silicate containing a chemically combined halogen. By the expression I mean lead-silicates consisting preponderantly of chemically combined lead and silicon oxides, a smaller amount'of a halogen, no oxides other than lead and silicon and no other ingredients except it be traces of impurities derived from raw materials or equipment used in the process.

In the operation of my process I can use either litharge (PbO) or other compounds of lead which upon heating supply to the product only PbO, such, for example, as red lead (Pbaoi) lead peroxide (H102) and white l'ead (ZPbCOaPbHaOz). A pure grade of fine-grained silica is suitable. The silica may be either natural or artificially prepared and ineither hydrous or anhydrous form. As the'accelerating stabilizing reagent I can use either the free hydroacids of the halogen elements such as hydrochloric and hydrofluoric acids or other halogen compounds, such as ammonium halide salts, which by reaction durin the process supply to the product only the halogen.

I use the accelerating-stabilizing halogen in the proportion of 0.2 to 5% based on the combined weight of lead oxide (PhD) and silica. Amounts as small as 0.1% are effective in stabilizing the product but do not greatly accelerate the reaction. Preferably the halogen should not be in excess of 5%, for, while larger amounts are efleetive they unnecessarily change the chemical composition of the pigment, reducing the percentage of lead silicate, the characteristic properties of which it is desired to retain.

I first thoroughly mix and mill the materials 7 l in the required proportions thereby reducing particle size and assuring a close association of ingredients. A porcelain or silex mill lining is satisfactory to avoid contamination of materials. In

case a solid acceleratonsuch as an ammonium halide salt, is used, the charge may be dry-milled but preferably I first wet-mill the silica and lead compounds together and then add the halogen compound as a solution and continue milling for a short time thus forming a very homogeneous mixture. When the halogen compound is added as a solution the halogen combines immediately and'completely with the lead compound. If the halogen compound is a solution of an ammonium salt, ammonia is liberated. The slurry discharged from the mill may be partially dewatered and dried if desired.

Either the wet pulp or the dry mixture is then charged into the heating furnace. A furnace lining of high-grade fire brick is satisfactory to avoid contamination of materials. Means should be provided for control of temperature. In furnaces where products of combustion contact the charge a neutral or oxidating flame should be maintained to avoid possible reduction of metallic oxides.

If the mixture of lead, silicon and halogen compounds has been prepared by dry-milling or blending, the reaction between the halogen and lead compounds takes place when the mixture is heated, and in case the halogen compound is an ammonium salt ammonia is evolved. The reaction between the halogen compound and lead oxide takesplace at low temperatures and is rapid and complete before the charge has reached a temperature at which leadv oxide and silica combine.

Lead oxide and sflica react very slowly at a temperature of 500 C. but at 550 C. a white lead silicate pigment can be made by heating for a period of about 48 hours. While such temperatures are obviously impractical for making lead silicates from mixtures of lead compounds and silica only, even lower temperature can be used.

if an accelerating and stabilizing compound of the class before referred to is used. By using accelerators my lead silicate pigments can be made in the temperature range 400 to 725 C. Preferred temperature ranges for the different types of lead silicates with variations in amount and kind of reaction accelerators will be shown later by examples.

The time required in the calcining zone of the furnace depends upon the temperature and amount of accelerator used: the higher the temperature within the permissible range the shorter the time. The time required to complete the reaction may vary from minutes to several hours but it is 'usually desirable to hold the charge at the proper temperature a little longer than appears absolutely necessary to insure that the reaction is complete. The progress of the reaction during calcination may be Judged by withdrawing a sample from the furnace for inspection. The finished product should be substantially white and should not acquire a yellower or darker color upon milling or grinding the sample in a mortar.

Lead silicate pigments, if not calcined at too high a temperature, are soft in texture and fine enough to be used as pigments for paint, but usually milling is desirable to break down asgregates and develop full tinting strength and hiding power of the products. Either dry or wetmilling methods may be used. Preferably the calcined product is wet-milled and the slurry dewatered and dried.

My methodof stabilizing lead silicate pigments and accelerating the reaction of their formation can be ap lied to essentially pure lead silicates of any desired composition. Not only can I make stable lead silicate pigments approximating the re ular molecular proportions such as subnormal lead silicate, PbSlzOs, normal lead silicate, PbSiCh; and basic lead silicate, PbsSiOi, but I can also make equally useful lead silicates with any basicity desired between the regular normal and basic formulae or of any subnormal composition desired between the regular subnormal and normal'formulae. I can also make useful lead silicate pigments containing a higher percentage of silica than the regular subnormal formula, PbSizOs. In case, an excess of either lead oxide or silica is used. such excess may be chemically or physically combined or, possibly, partially in the free state. By the expressions normal lead sillcate, subnormal lead silicate and basic lead silicate, as used in this specification and claims, I mean by normal lead silicate, a product having an equimolecular ratio of PbO to SlOz, by subnormal lead silicate, a product having a ratio of PbOto $102 less than equimolecular and by basic lead silicate a product having a ratio of PM) to SiOz greater than equimolecular. Lead silicate pigments other than the normal PbSiOa, may contain. and some of them probably do contain, more than one definite chemical compound, the various silicates, basic, normal and subnormal,

adjusting during the heating process proportionally in the product in accordance with natural laws to the state of most stable equilibrium.

- The mechanism of the reactions whereby small amounts of the halogen compounds accelerate or catalyze the reaction in the formation of lead silicates and stabilize the resulting products is not definitely known, but some observations in the operation of the process and examination of the products indicate the nature of the combination of the halogen in the product. Rapid and complete combination of the halogen with the lead compound, as already explained, is confirmed by the fact that when the halogen compound is a free acid, for example hydrochloric acid, and mixed by wet-milling, the water separated from the solids is neutral to litmus notwithstanding the fact that normal lead chloride has an acid reaction and is appreciably soluble in water. Furthermore, the water contains no lead or chlorine. After the mixture has been calcined treatment of the produ'ct with water fails to extract any lead halide, although analyses have shown that substantially all the halogen added remains in the product. The finished pigment is neutral to litmus or' possibly in some cases has a slightly alkaline reaction. These observations indicate that the halide is in fixed chemical combination, possibly as a highly basic lead halide.

The examples in the following table illustrate the working of my process. The charges were all proportioned to yield essentially pure normal lead silicates, using 78.80 parts of lead oxide (PbO) to 21.20 parts of silica. The nature and amount of accelerating-stabilizing halogen compound added is shown in column 2 of the table. The mixture in Example 2 was dry-milled: in all the other examples the mixtures were wet-milled and Partially dried before transferring to the calcining furnace. The mixtures were calcined under the conditions shown in columns 3 and 4 of which were tested for completeness of reaction as before described. Column 5 shows the tinting strength of the products compared with their white color when exposed to sunlight.

Ex 7 Time Tinting m Reaction accelerator-per cal- Temperastrength, cent of mixture cined, turc, "0. white hours lead- 1.. None'used 4.8 650 2 ii ammonium chloride... 2 480 to 495 180 8 3 hydrochloric acid l 490 to 610 100 4 do 0.25 650 6 1.5 hydrochloric acid 4 550 110 6 1 ammonium fluoride... 7 400 to 510 l 1. 0.5 7 ammonium iiuoride.. 2 620 to 650 l 8. 0.26 ammonium fluoride 5 490 to 610 180 9 1 ydroiiuoric acid 4 450 to 470 ll ammonium bromide..- 3 550 150 t a ammonium iodide 3 650 standard white lead taken as 100. The products in all cases except that of Example 1 retained The results in the foregoing table show that in all cases where a halogen has been added the time required for reaction is greatly reduced in comparison with Example .1 in which no accele'ratorwas used. It is also clear that the use of the accelerators permits operation at lower temperatures. This fact is particularly important since the results in the table show clearly that, in general, the lower the calcining temperature the higher the tinting strength of the product.

The use of the stated halogen compounds, having the dual function of accelerating the reaction and stabilizing the reactionproduct, permits'considerable variation in the calcining operation. Thus it is possible to complete the reaction between the lead compoundand silica-in the minimum time with relatively more of the halogen compound or with a similar amount to complete the reaction at a lower temperature in a somewhat longer period of time. On the other hand, the pigment may be stabilized with very small amounts of halogen compounds, the time for completion of the reaction depending on the temperature, and thus yielding a stabilized lead sillcate pigment having a minimum content of the stabilizing compound.

The following examples, l2, l3, l4 and 15, show the application of my process in the preparation of lead silicate pigments having compositions other than normal compound. Example 16 illushates the use in the process oi lead compounds other than lead oxide (PhD).

Example 1.2.Su2mormal lead silicate A mixture of 206 parts of lead oxide (P110) and its parts of silica was wet-milled for 2.5 hours.

3:75 parts of actual hydrochloric acid (H01), canal to 1% oi the total of lead oxide and silica,

diluted with Zilil parts of water was then added and milling continued to hour lower.

The charge was washed from the mill with a little water, settled, the liquid decanted and the siurry'was dried at about 100 C.

The dry mixture was calcined for 2 hours in the temperature range etc to 500 C.

The calcined product was wet-milled for 2 hours then clewatered and dried at about 100 C.

. Percent chlorine found by analysis--- 6.91 ffompoeltion of pigment:

Lead oxide (PbO) 54.50

Silica 44.59

Chlorine 13.91

Tinting strength, liO compared with a standard white lead taken as 100.

Example Z3.Suhno1mal lead silicate Per cent Combined fluorine found by analysis---" 0.45 Composition of pigment:

Lead oxide (PbO) 69.68

Silica 29.87

Fluorine 0.45

Tinting strength, 125 compared with a standard white lead taken as 100.

Example 15'.Basic lead silicate A mixture of '85 parts of lead oxide (PhD) and 15 parts of silica was wet-milled with a solution of 5 parts of ammonium chloride. The slurry was .dewatered by settling and decantation and the solids dried.

The mixture was then calcined for 3 hours at 570 to 590 C.

The calcined product had a slightly creamwhlte color and was very faintly alkaline to litmus.

. Per cent Chlorine found by analysis 3.03

Qomposition of pigment:

Lead oinde (PcO) 82.43

Silica $14.54

Chlorine 3.03

The finished pigment had a tinting strength of 3.75 compared with a standard white lead taken no mo tinting strength.

Example id-Subatomic! lead silicate ports. of ammonium fluoride, equal to' 1% oi the opaque, white lead silicate pigment which com- 1 Tinting strength, compared with a standard white lead taken as 100.

Example 162-4107112121 lead silicate 4.5? parts of white lead (ZPbCOaJPbI-IcOz), equal to 3.94 parts of lead oxide (PbO), 1.06 parts of silica and 0.25 part of ammonium chloride, equal 13069;, of the calculated weight of PM) and silica, were dry-ground to an intimate mixture.

The mixture was then calcined for 5 hours in the temperature range 509 to 551 C.

The calcined pigment had a cream white color and a tinting strength oi 120 compared with a standard white lead taken as 108.

Two classes of white pigments are generally (l) The opaque white pigments, such lead, zinc oxideand titanium pigments as whi which impart opacity and whiteness when mixed with oil or other organic vehicle: and (2) The extender pigments, such as whiting and china clay which do not impart appreciable opacity when similarly mixed with vehicles. My lead silicate pigments belong to the class of opaque white pigments and their properties adapt them for general application in oil, enamel and lacquer- Wpe paints and for other purposes where white pigments are used, such as for rubber,-

linoleum and other floor coverings, printing inks, vitrified enamel et cetera but I have found them particularly desirable ion one preparation of exterior paints. Paints made'wlth my lead silicate pits ground with raw linseed oil vehicle are exceptionally durable when applied to either wood or metal surfaces and exposed the weather.

I claim as my invention: 1. The method of making an essentially pure,

prises intimately mixing compounds which provide, on subsequent heating, only PbO, silica and a halogen, said halogen being in combination with a cation which is volatilized at the temperature of heating, and then heating the mixture at a temperature suiflciently high to chemically combine the lead oxide, silica and halogen in the solid phase without fusion to yield a lead silicate pigment containing said halogen in amount not greater than 5% of the total weight of PbO and silica.

2. The method of making an essentially pure, opaque, white lead silicate pigment which comprises intimately wet-mixing compounds which provide, on subsequent heating, only PbO, silica and a halogen, said halogen being in combination with a cation which is volatilized at the temperature of heating, and then heating the mixture at a temperature sufllciently high to chemically combine the lead oxide, silica and halogen in the solid phase without fusion to yield a lead silicate pigment containing said halogen in amount not greater than 5% of the total weight of PhD and silica.

3. The method of making an essentially pure,

opaque, white lead silicate pigment which comprises intirnately mixing compounds which provide, on subsequent heating, only PbO, silica and a halogen, said halogen being in combination wlth'a cation which is volatilized at the temperature of heating, and then heating the mixture in the temperature range of 400 to 750? C. to chemically combine the lead oxide, silica and halogen in the solid phase without fusion to yield a lead silicate pigment containing said halogen in amount not greater than 5% of the total weight of PhD and silica. 4. The method of making an essentially pure, opaque, white, normal lead silicate pigment which comprises intimately mixing compounds which provide, on subsequent heating, only PbO, silica and a halogen, said halogen being in combination with a cation which is volatilized at the temperature of heating, and then heating the mixture at a temperature sufliciently high to chemically combine the lead oxide, silica and halogen in the solid phase without fusion to yield a normal lead silicate pigment containing said halogen in amount not greater than 5% of the total weight of PhD and silica.

5. The method of making an essentially pure, opaque, white, subnormal lead silicate pigment which comprises intimately mixing compounds which provide, on subsequent heating, only PbO, silica and a halogen, said halogen being in combination with a cation which is volatilized at the temperature of heating, and then heating the mixture at a temperature sufllciently high to chenilcally combine the lead oxide, silica and halogen in th'esolid phase without fusion to yield a subnormal lead silicate pigment containing said halogen in amount not greater than 5% of the total weight of PbO and silica.

6. The method of making an essentially pure,

7. The method of making an essentially pure. opaque, white lead silicate pigment which comprises intimately mixin compounds which provide, on subsequent heating, only PbO, silica and chlorine, the chlorine being in combination with a cation which is volatilized at the temperature of heating, and then heating the mixture at a temperature sufficiently high to chemically combine the lead oxide, silica and chlorine in the solid phase without fusion to yield a lead silicate pigment containing chlorine in amount not greater than 5% of the total weight of PhD and silica.

8. The method of making an essentially pure. opaque, white lead silicate pigment which comprises intimately mixing compounds which provide, on subsequent heating, only PbO, silica and fluorine, the fluorine being in combination with a cation which is volatilized at the temperature of heating, and then heating the mixture at a temperature sufficiently high to chemically combine the lead oxide, silica and fluorine in the solid phase without fusion to yield a lead silicate pigment containing fluorine in amount not greater than 5% of the total weight of PbO and silica.

9. The method of making an essentially pure, opaque, white lead silicate pigment which comprises intimately mixing compounds which provide, on subsequent heating, only PbO, silica and iodine, the iodine being in combination with a cation which is volatilized at the temperature of heating, and then heating the mixture at a temperature sufficiently high to chemically combine the lead oxide, silica and iodine in the solid phase without fusion to yield a lead silicate pig-. ment containing iodine in amount not greater than 5% of the total weight of PhD and silica.

10. An essentially pure, water-insoluble, lightstable, opaque white lead silicate pigment comprising chemically combined lead oxide, silica and halogen, said halogen being in amount not in excess of 5% of the total weight of lead and silicon calculated as PbO and silica, formed by heating together, at a temperature sufllciently high to combine the lead oxide, silica and halogen in the solid phase without fusion, a mixture of compounds which provide on heating only lead oxide. silica and halogen, said halogen being in combination with a cation which is volatilized at the temperature of heating.

11. An essentially pure, water-insoluble, lightstable, opaque white lead silicate pigment comprising chemically combined lead oxide, silica and chlorine, said chlorine being in amount not in excess of 5% of the total weight of lead and silicon calculated as PhD and silica, formed by heating together, at a temperature sufliciently high to combine the lead oxide, silica and chlorine in the solid phase without fusion, a mixture of compounds which provide on heating only lead oxide, silica and chlorine, said chlorine being in combination with a cation which is volatilized at the temperature of heating.

12. An essentially pure, water-insoluble, lightstable, opaque white lead silicate pigment comprising chemically combined lead oxlde, silica and fluorine, said fluorine being in amount not in excess of 5% of the total weight of lead and silicon calculated as PbO and silica, formed by heating together, at a temperature sumciently high to combine the lead oxide, silica and fluorine in the solid phase without fusion, a mixture of compounds which provide on heating only lead oxide, silica and fluorine, said fluorine being in combination with a cation which is volatilized at the temperature of heating.

in: together, at a temperature suiilciently high LOUIS EBARTON.

13. An essentially pure, water-insoluble, lightto I mbine the lead oxide, silica and iodine in the stable, opaque white lead silicate pigment comsolid phase without fusion, a'mixture of com-1 prising chemically combined lead oxide, silica pounds which provide onheating only lead oxide, and iodine, said iodine being in amount not in silica and iodine, said iodine being in combinaexcess of 5% o! the total weight of lead and tion with a. cation which is volatilized at the tenfcon calculated as PbO and silica. formed by heatperature of heating. 

